Resistive sensor structures are known in the art. Two well-known examples are: a Wheatstone bridge and a Hall element.
Wheatstone bridges are used for example in pressure sensors, and the basic principles of using piezo-resistive elements and circuits for biasing and reading the bridge structure are known in the art for many decades, see for example GB1547592 (published in 1979).
Hall elements for measuring a magnetic field are also very well known, and are used inter alia in current sensors, or in angular position sensors, where a magnetic field (e.g. generated by a permanent magnet) is measured at several locations of the sensor device, and is converted into an angular position, as described for example in WO9854547 (published in 1998).
However, Wheatstone bridges and Hall elements are passive resistive structures, and need to conduct a current before an output-signal can be retrieved from them. The term “biasing” is used for applying a voltage or current to such a structure. The term “readout” is used for retrieving a sensor signal or a sensor value from such a structure.
In both cases the “direct output signal” of the passive resistive structure is a differential voltage signal ΔV, but for the examples given above, the actual physical value to be measured is “a pressure” and “an angular position”. This requires another conversion step, which may involve a multiplication with a constant factor in case of a pressure sensor, and/or may involve measurement of two or more magnetic field values, and goniometric operations, for example. Such conversions are known in the art, but not the focus of the present invention, and hence will not be further discussed herein.
The present invention is concerned with obtaining accurate values from the passive resistive sensor structures. One problem with passive resistive structures is known as “zero offset”. This problem is addressed in the field inter alia by a technique known as “chopping” if the DC value of the sensor signal is not relevant, and in the case of Hall elements, by a technique known as “spinning current”. But these techniques do not solve all inaccuracies, in particular problems related to mechanical stress and/or temperature variations and/or due to voltage variations.
In order to better understand and appreciate the present invention, first the classical ways of biasing and reading out a Hall element will be described referring to FIG. 1(a) to FIG. 1(f), and for a Wheatstone bridge with reference to FIG. 2.
A basic Hall element (also referred to as “Hall plate”) consist of a conducting material provided with at least four electrical contacts. In order to make use of the “Hall effect”, a current has to flow through the element. A bias current I is supplied via two of the contacts, which will be referred to as the “current nodes” or “excitation nodes” or “input nodes”. Two other contacts, referred to as the “sense contacts” or “readout nodes” or “output nodes”, are typically placed on an equipotential line, to make the voltage difference between the sense contacts zero in the absence of a magnetic field. The principle of measuring a magnetic field component Bz using a horizontal Hall element is illustrated in FIG. 1(a). For a Hall readout, the current contacts A, C and sense contacts B, D alternate with each other. If an excitation current Iex is applied to the current contacts A, C, and if an out-of-plane magnetic field Bz is applied to the element, a Hall voltage VH proportional to the applied magnetic field Bz will appear between the sense contacts B, D; in other words, VH=VB−VD.
There are two common approaches for realizing the biasing current. One approach uses a current source, in which case the nominal value of the applied current I is known (I=Iex). A possible implementation of this case is shown in FIG. 1(b). The other approach uses a voltage source, in which case the nominal voltage over the plate is known (V=Vex). An implementation of this type is shown in FIG. 1(c). It is to be noted that, where in the above the nominal value of the bias source is said to be known, the actual value deviates from this nominal (or expected) value through various mechanisms. This can be due to ageing, or due to environmental influences such as temperature dependencies, parasitic stress (e.g. from the package), etc. Considered over the lifetime of a sensor, the bias source parameters drift from their expected (nominal) value.
When applying “current-biasing”, the voltage over the plate is not exactly known, but depends on the electrical resistance of the plate. In the case of “voltage-biasing”, the current flowing through the plate is not exactly known, but is determined by the electrical resistance of the plate. This electrical resistance of the Hall plate varies with temperature and stress (e.g. through piezo-resistive effects), and constitutes another source of drift. The sensor resistance also affects the dynamic response of the sensor structure to changes in the applied biasing, as is needed when applying the “spinning current technique”.
FIG. 2 shows a biasing circuit connected to a Wheatstone-bridge. In the example shown, the biasing circuit applies a constant voltage Vdd to the excitation nodes A, C. In a manner similar to FIG. 1, the differential output ΔVout of the bridge can be measured over the output nodes B, D. While not shown in FIG. 2, it is also possible to inject a constant current to the excitation nodes A, C, and to readout the voltage Vout over the nodes D, B. The real sensor signal, meaning the value to be actually measured by the sensor structure, for example pressure exerted on a membrane, is then a function of the output signal ΔVout and Vdd, but also of other factors such as temperature, package stress etc. While the relation between the output voltage ΔVout of the sensor structure and the actual physical signal to be measured (e.g. pressure) can be approximated very well by a linear relation, using a constant factor known as “sensitivity of the sensor”, in practice the sensitivity is not exactly constant, and needs to be corrected for temperature variations and/or packaging stress.
For the considered sensors, drift of the biasing source causes drift in the sensitivity of the sensor. For a Hall sensor, also piezo-resistive effects of the sensor structure itself may be a source of drift. The total drift of a sensor needs to be kept within bounds over its lifetime, which may require compensation.
Sensors with a Wheatstone bridge are still being developed in recent years, but the focus has shifted to more accurate sensors, for example sensors having a reduced sensitivity to stress and/or temperature, for example as described in EP0736756 (published in 1996) focusing mainly on a special arrangement of the piezo-resistive elements, or in EP3032235 (published in 2016) focusing on both a special arrangement of the piezo-resistive elements and on the biasing circuit.
Likewise, sensors with one or more Hall elements are still being designed in recent years, and also for this type of sensors, large efforts are done to improve accuracy and to make the sensitivity less dependent on external influences. One problem with Hall elements, known as “offset-error, is being addressed by the well-known and widespread “spinning current technique”, but significant efforts are being made to reduce also other influences, in particular stress and temperature influences, as described for example in EP3109658 (published 28 Dec. 2016).